I had a hard fight with iTunes yesterday, but eventually sorted it out.
The dialog below is just one of many bugs and awful usability problems I run into. Among software that I use, iTunes literally ranks dead last on quality and usablility. Will the new iTunes coming in macOS Catalina fix these problems or make them worse?
The first issue is that there are/were a dozen or so "1a" and "8a" tracks, but iTunes does not say which, forcing a manual search.
The second issues is that the explanatin of the problem is flat-out wrong. Indeed, tracks 1a and 8a were both the files I selected to instruct iTunes as to the new location. It then found the other 800 or so tracks, but not the tracks I explicitly pointed it to!
When I finally figured out which 1a and 8a tracks it was unhappy about, I had to manually remove and re-add these tracks. There never was any issue with the tracks, just a bug in iTunes. This is just one of a number of bugs I’ve run into in iTunes.